Modified lithium iron phosphate secondary particle material, preparation method thereof, positive pole piece and application of positive pole piece

Abstract

The invention relates to the technical field of batteries, and discloses a modified lithium iron phosphate secondary particle material and a preparation method thereof, and a positive pole piece and application thereof. The modified lithium iron phosphate secondary particle material contains a lithium iron phosphate secondary particle material and a carbon nanomaterial growing inside and on the surface of the lithium iron phosphate secondary particle material, and the lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a non-uniform coefficient Cu less than 3.2. Carbon nanomaterials grow inside and on the surface of the modified lithium iron phosphate secondary particle material, so that the conductivity of the lithium iron phosphate secondary particle material is improved, and meanwhile, the specific surface area of the modified lithium iron phosphate secondary particle is increased.

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and in particular to a modified lithium iron phosphate secondary particle material and a preparation method thereof, a positive electrode sheet and applications thereof. Background Art

[0002] During the electrode design process, a reasonable electrode structure can be constructed by a double-layer electrode. The use of primary particles and secondary particles in layered coating is an important way to construct a reasonable electrode structure. The upper layer uses high-porosity secondary particle materials to improve Li + Liquid phase diffusion capacity; the lower layer uses high-density, low-porosity primary particle materials to increase the energy density of the battery cell. This electrode structure can simultaneously achieve high density and a pore structure that is conducive to ion diffusion.

[0003] However, the conductivity of the particles inside the existing secondary particle materials is relatively poor. Therefore, there is an urgent need to optimize the lithium iron phosphate secondary particle materials to improve their conductivity and thus enhance the kinetic performance of the battery. Summary of the Invention

[0004] The present invention aims to overcome the problem of poor conductivity in existing secondary granular materials, where the internal particles of the materials lack sufficient contact with the conductive agent. The present invention provides a modified lithium iron phosphate secondary granular material, a preparation method thereof, a positive electrode plate, and applications thereof. The modified lithium iron phosphate secondary granular material has carbon nanomaterials grown within and on the surface of the particles, improving their conductivity and increasing the specific surface area of the modified particles.

[0005] In order to achieve the above object, the present invention provides a modified lithium iron phosphate secondary particle material, which comprises a lithium iron phosphate secondary particle material and a carbon nanomaterial grown inside and on the surface of the lithium iron phosphate secondary particle material, wherein the lithium iron phosphate secondary particle material is composed of an uneven coefficient C u The material is composed of lithium iron phosphate primary particles with a density of less than 3.2.

[0006] Optionally, based on the total weight of the modified lithium iron phosphate secondary particle material, the content of the carbon nanomaterial is 0.15 to 2 wt%, preferably 0.3 to 0.9 wt%.

[0007] Optionally, the modified lithium iron phosphate secondary particle material has a porous structure and a specific surface area of 13.5 to 15.7 m 2 / g.

[0008] Optionally, the particle size D50 of the lithium iron phosphate secondary particle material is 6 to 9 μm

[0009] Optionally, the carbon content in the modified lithium iron phosphate secondary particle material is 1-3 wt %.

[0010] Optionally, the carbon nanomaterial is carbon nanotubes and / or carbon nanoparticles.

[0011] Optionally, the carbon nanotubes have a diameter of 50 to 200 nm.

[0012] Optionally, the carbon nanoparticles have a diameter of 15 to 40 nm.

[0013] Optionally, the resistivity of the modified lithium iron phosphate secondary particle material is 3 to 6 Ω·cm.

[0014] The second aspect of the present invention provides a method for preparing a modified lithium iron phosphate secondary particle material, the method comprising: pyrolyzing the lithium iron phosphate secondary particles and a carbon source under an inert atmosphere,

[0015] Wherein, the lithium iron phosphate secondary particle material is composed of a non-uniform coefficient C u <3.2 lithium iron phosphate primary particle material;

[0016] The pyrolysis temperature is 600-800° C., and the mass ratio of the lithium iron phosphate secondary particles to the carbon source is 5:1-3.

[0017] Optionally, the pyrolysis time is 5 to 8 hours.

[0018] Optionally, the carbon source is melamine.

[0019] The third aspect of the present invention provides a modified lithium iron phosphate secondary particle material prepared by the method described above.

[0020] The fourth aspect of the present invention provides a positive electrode plate, which includes a first positive electrode active material layer and a second positive electrode active material layer, the first positive electrode active material layer contains the modified lithium iron phosphate secondary particle material described above, and the second positive electrode active material layer contains lithium iron phosphate primary particle material.

[0021] Optionally, the positive electrode sheet further includes a positive electrode current collector, and the second positive electrode active material layer and the first positive electrode active material layer are sequentially covered on one side of the positive electrode current collector.

[0022] Optionally, the single-sided density of the positive electrode sheet is 180 to 300 m 2 / g, compacted density is 2~2.8g / cm 3 .

[0023] A fifth aspect of the present invention provides a battery, which includes the positive electrode plate described above.

[0024] A sixth aspect of the present invention provides an electrical device, which includes the battery described above.

[0025] Compared with the prior art, the present invention has at least the following advantages:

[0026] (1) The modified lithium iron phosphate secondary particle material provided by the present invention has carbon nanomaterials grown inside and on the surface of the lithium iron phosphate secondary particle material, and the lithium iron phosphate secondary particle material is composed of an uneven coefficient C u The modified material is composed of a specific lithium iron phosphate primary particle material. The conductivity of the lithium iron phosphate secondary particle material is improved by the modified material, and the specific surface area of the modified lithium iron phosphate secondary particles is increased, which is beneficial for the lithium iron phosphate secondary particles to absorb the electrolyte and reduce the liquid phase diffusion impedance of the positive electrode active material.

[0027] (2) The method of the present invention is simple to operate. It only needs to u By pyrolyzing the lithium iron phosphate secondary particle material composed of a specific lithium iron phosphate primary particle material and a carbon source at a specific temperature and in a specific dosage ratio, a carbon nanomaterial with a specific content grown inside and on the surface of the lithium iron phosphate secondary particle material can be obtained, thereby obtaining a modified lithium iron phosphate secondary particle material with excellent conductivity and a large surface area, and this method will not introduce other impurities.

[0028] (3) The positive electrode sheet provided by the present invention includes two layers of active material. The first positive electrode active material layer uses modified lithium iron phosphate secondary granular material as the active material, which can improve its conductivity and specific surface area and provide good ion and electron pathways. The second positive electrode active material layer uses lithium iron phosphate primary granular material as the active material, which has high compaction capability. In summary, this positive electrode sheet has the advantages of high compaction, excellent pore structure, and good conductivity.

[0029] (4) The battery containing the positive electrode sheet has improved battery kinetic performance while ensuring a higher energy density. DETAILED DESCRIPTION

[0030] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0031] The endpoints of the ranges and any values disclosed herein are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0032] A first aspect of the present invention provides a modified lithium iron phosphate secondary particle material. The modified lithium iron phosphate secondary particle material comprises a lithium iron phosphate secondary particle material and a carbon nanomaterial grown inside and on the surface of the lithium iron phosphate secondary particle material.

[0033] In the present invention, the carbon nanomaterial may be present on the surface of the lithium iron phosphate secondary particle material or between the gaps between the particles inside the lithium iron phosphate secondary particle material. The carbon nanomaterial grown inside and on the surface of the lithium iron phosphate secondary particle material may be in various forms commonly found in the art. In some embodiments, the carbon nanomaterial may be a carbon nanotube and / or carbon nanoparticle, preferably a carbon nanotube. In a more preferred embodiment, the diameter of the carbon nanotube is 50 to 200 nm. In another more preferred embodiment, the diameter of the carbon nanoparticle is 15 to 40 nm.

[0034] In the modified lithium iron phosphate secondary particle material of the present invention, the lithium iron phosphate secondary particle material is composed of a non-uniformity coefficient C u The non-uniformity coefficient C is less than 3.2. u The internal pores of the lithium iron phosphate secondary particle material composed of the lithium iron phosphate primary particle material with a uniformity coefficient C < 3.2 are conducive to the growth of carbon nanomaterials. u =D60 / D10, wherein D60 refers to the particle size distribution curve that is smaller than 60% of the total volume, and D10 refers to the particle size distribution curve that is smaller than 10% of the total volume.

[0035] In a preferred embodiment, the particle size D50 of the lithium iron phosphate secondary granular material is 6 to 9 μm. When D50 is too small, the electrode liquid phase diffusion impedance of the positive electrode plate increases; when D50 is too large, the compaction ability of the modified lithium iron phosphate secondary granular material decreases.

[0036] Resistivity testing revealed that the modified lithium iron phosphate secondary granular material exhibited a significantly lower resistivity than the unmodified lithium iron phosphate secondary granular material, demonstrating that the modified lithium iron phosphate secondary granular material provided herein exhibits effective improvement in conductivity. In some embodiments, the resistivity of the modified lithium iron phosphate secondary granular material may be 3-6 Ω·cm, for example, 3 Ω·cm, 3.5 Ω·cm, 4 Ω·cm, 4.5 Ω·cm, 5 Ω·cm, 5.5 Ω·cm, or 6 Ω·cm.

[0037] In some embodiments, the content of the carbon nanomaterial can be 0.15-2wt% based on the total weight of the modified lithium iron phosphate secondary granular material, such as 0.15wt%, 0.2wt%, 0.25wt%, 0.3wt%, 0.35wt%, 0.4wt%, 0.45wt%, 0.5wt%, 0.55wt%, 0.6wt%, 0.65wt%, 0.7wt%, 0.75wt%, 0.8wt%, 0.85wt%, 0.9wt%, 1%, 1.5% or 2wt%. In a preferred embodiment, the content of the carbon nanomaterial can be 0.3-0.9wt% based on the total weight of the modified lithium iron phosphate secondary granular material. Within this preferred range, the carbon nanomaterial in the modified lithium iron phosphate secondary granular material can greatly improve the electrical conductivity of the lithium iron phosphate secondary granular material, thereby effectively improving the electrical conductivity of the electrode.

[0038] In the present invention, the method for determining the content of carbon nanomaterials in the modified lithium iron phosphate secondary particle material is as follows: the carbon content in the lithium iron phosphate secondary particle material before modification and the modified lithium iron phosphate secondary particle material are tested by thermogravimetry, and the difference between the two is calculated, which is the content of carbon nanomaterials in the modified lithium iron phosphate secondary particle material.

[0039] The lithium iron phosphate secondary particle material is composed of lithium iron phosphate primary particle material. The lithium iron phosphate primary particle material group is usually a lithium iron phosphate material coated with carbon. Therefore, the carbon content in the modified lithium iron phosphate secondary particle material is not equal to the content of the carbon nanomaterial in the modified lithium iron phosphate secondary particle material, and the carbon content in the modified lithium iron phosphate secondary particle material is the total carbon content in the modified lithium iron phosphate secondary particle material, specifically the total carbon content of the carbon coating layer of the unmodified lithium iron phosphate secondary particle material itself and the carbon nanomaterial grown after modification.

[0040] In some embodiments, the carbon content in the modified lithium iron phosphate secondary particle material can be 1 to 3 wt%, for example, it can be 1 wt%, 1.5 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.5 wt% or 3 wt%.

[0041] Compared with the lithium iron phosphate secondary particle material before modification, the morphology of the modified lithium iron phosphate secondary particle material of the present invention has not changed significantly, but the specific surface area has increased by about 0.8 to 1.5%. In some embodiments, the modified lithium iron phosphate secondary particle material has a porous structure and a specific surface area of 13.5 to 15.7 m 2 / g. After the specific surface area of the modified lithium iron phosphate secondary particle material is increased, the Li + Liquid phase diffusion capacity and electron transport capacity.

[0042] In the present invention, the lithium iron phosphate secondary particle material is composed of multiple lithium iron phosphate particles, and has a relatively large porous structure inside, which can provide a path for lithium ion transmission and is a good upper electrode active material. However, the lithium iron phosphate particles inside the lithium iron phosphate secondary particles cannot directly contact the conductive agent. The modified lithium iron phosphate secondary particle material provided by the present invention can improve the conductivity of the lithium iron phosphate secondary particle material after the internal growth of carbon nanomaterials, and the specific surface area of the lithium iron phosphate secondary particle material is increased, which is conducive to the electrode adsorbing more electrolyte and improving the dynamic performance of the battery.

[0043] A second aspect of the present invention provides a method for preparing a modified lithium iron phosphate secondary particle material, the method comprising: pyrolyzing the lithium iron phosphate secondary particles and a carbon source under an inert atmosphere.

[0044] In the method described in the present invention, the lithium iron phosphate secondary particles are mixed with a carbon source and pyrolyzed, and the carbon atoms in the carbon source enter the interior of the lithium iron phosphate secondary particle material to form carbon nanomaterials, thereby obtaining a modified lithium iron phosphate secondary particle material.

[0045] In the method described in the present invention, the lithium iron phosphate secondary particle material is composed of a non-uniform coefficient C u <3.2 lithium iron phosphate primary particle material. When the non-uniformity coefficient C u When the nonuniformity coefficient C is too large, the particles are more densely packed, which is not conducive to the growth of carbon nanomaterials. u If the temperature is too small, the compaction density of the lithium iron phosphate secondary particle material will be too low, affecting the battery capacity.

[0046] In some preferred embodiments, the particle size D50 of the lithium iron phosphate secondary particle material used to prepare the modified lithium iron phosphate secondary particle material is 6 to 9 μm.

[0047] In the present invention, the content of carbon nanomaterials in the modified lithium iron phosphate secondary particle material can be controlled by adjusting the pyrolysis temperature. When the pyrolysis temperature is too low, carbon nanomaterials cannot be formed inside the lithium iron phosphate secondary particle material; when the pyrolysis temperature is too high, the carbon nanomaterials may decompose. In a preferred embodiment, the pyrolysis temperature can be 600-800°C, for example, 600°C, 650°C, 700°C, 750°C or 800°C. By controlling the pyrolysis temperature within a range, a modified lithium iron phosphate secondary particle material with a carbon nanomaterial content of 0.15-0.75wt% can be obtained, so that the prepared modified lithium iron phosphate secondary particle material has both high conductivity and specific surface area.

[0048] In some embodiments, the pyrolysis time may be 5 to 8 hours, for example, 5 hours, 5.5 hours, 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours.

[0049] In the present invention, if the ratio of the amount of the lithium iron phosphate secondary particle material to the carbon source is too large, the amount of the carbon source is too small, the carbon nanomaterial formed inside the modified lithium iron phosphate secondary particle material is small, and the conductive performance is poor; if the ratio of the amount of the lithium iron phosphate secondary particle material to the carbon source is too small, the amount of the carbon source is too large, and the carbon nanomaterial formed inside the modified lithium iron phosphate secondary particle material is large, resulting in a decrease in the specific surface area of the modified lithium iron phosphate secondary particle material, affecting the Li + Liquid phase diffusion capacity and electron transport capacity. In a preferred embodiment, the mass ratio of the lithium iron phosphate secondary particles to the carbon source can be 5:1 to 3, for example, 5:1, 5:1.5, 5:2, 5:2.5, or 5:3. By controlling the mass ratio of the lithium iron phosphate secondary particles to the carbon source within this range, a modified lithium iron phosphate secondary particle material with high conductivity and specific surface area can be produced.

[0050] In the present invention, the carbon source is not limited, as long as it can achieve the purpose of the present invention. In a preferred embodiment, the carbon source can be melamine.

[0051] In the present invention, the inert atmosphere can be a conventional choice in the art. In some embodiments, the inert atmosphere includes but is not limited to nitrogen atmosphere and / or argon atmosphere.

[0052] The method of the present invention is simple and easy to operate, and can not only obtain a modified lithium iron phosphate secondary particle material with more excellent conductive properties, but also avoid the introduction of other impurities.

[0053] The third aspect of the present invention provides a modified lithium iron phosphate secondary particle material prepared by the method described above.

[0054] In a preferred embodiment, the modified lithium iron phosphate secondary particle material comprises a lithium iron phosphate secondary particle material and a carbon nanomaterial grown inside and on the surface of the lithium iron phosphate secondary particle material, wherein the lithium iron phosphate secondary particle material is composed of a non-uniform coefficient C u The material is composed of lithium iron phosphate primary particles with a density of less than 3.2.

[0055] In a preferred embodiment, based on the total weight of the modified lithium iron phosphate secondary particle material, the content of the carbon nanomaterial is 0.15-2 wt%, more preferably 0.3-0.9 wt%.

[0056] In a preferred embodiment, the modified lithium iron phosphate secondary particle material has a porous structure and a specific surface area of 13.5 to 15.7 m 2 / g.

[0057] In a preferred embodiment, the particle size D50 of the lithium iron phosphate secondary particle material is 6 to 9 μm.

[0058] In a preferred embodiment, the carbon content in the modified lithium iron phosphate secondary particle material is 1 to 3 wt %.

[0059] In a preferred embodiment, the carbon nanomaterial is a carbon nanotube and / or carbon nanoparticle. More preferably, the carbon nanotube has a diameter of 50 to 200 nm. More preferably, the carbon nanoparticle has a diameter of 15 to 40 nm.

[0060] In a preferred embodiment, the resistivity of the modified lithium iron phosphate secondary particle material is 3 to 6 Ω·cm.

[0061] The fourth aspect of the present invention provides a positive electrode plate, which includes a first positive electrode active material layer and a second positive electrode active material layer, wherein the first positive electrode active material layer contains the modified lithium iron phosphate secondary particle material described above, and the second positive electrode active material layer contains lithium iron phosphate primary particle material. In the positive electrode plate, the lithium iron phosphate primary particle material preferably has a non-uniformity coefficient C u Lithium iron phosphate primary particle material with a density of <3.2.

[0062] The positive electrode plate described in the present invention adopts a double-layer positive electrode active material layer design to construct a reasonable electrode structure, wherein the first positive electrode active material layer uses modified lithium iron phosphate secondary granular material as the active material, which can improve the conductivity and specific surface area of the positive electrode plate, and the modified lithium iron phosphate secondary granular material has a large number of pore structures inside, which can provide good ion and electron pathways for the positive electrode plate and reduce the battery impedance; the second positive electrode active material layer uses conventional lithium iron phosphate primary granular material as the active material, has a high compaction capability, and ensures the energy density of the positive electrode plate.

[0063] In a preferred embodiment, the positive electrode sheet further includes a positive current collector, and the second positive electrode active material layer and the first positive electrode active material layer are sequentially covered on one side of the positive current collector. In the positive electrode sheet, the second positive electrode active material layer close to the positive current collector has a high compaction density and a low porosity, which can ensure the electrode energy density; while the first positive electrode active material layer close to the separator can improve the Li + Liquid phase diffusion capacity and electron transport capacity. The electrode obtained by this design can simultaneously have high compaction density, excellent pore structure and good electrical conductivity.

[0064] In some embodiments, the single-surface density of the positive electrode sheet can be 180 to 300 m 2 / g, and the compacted density can be 2 to 2.8 g / cm 3 .

[0065] The positive electrode plate structure provided by the present invention takes into account the pore distribution in the thickness direction of the electrode and the influence of the conductive network on the electrode impedance. It is suitable for the construction of electrode structures in various materials and can also ensure that the battery has a high energy density while improving the battery's kinetic capabilities.

[0066] In one embodiment, the method for preparing the positive electrode sheet of the present invention includes:

[0067] (1) preparing a first slurry by mixing the modified lithium iron phosphate secondary particle material, a conductive agent and a binder;

[0068] (2) preparing a second slurry from lithium iron phosphate primary particle material, a conductive agent, and a binder;

[0069] (3) The second slurry and the first slurry are sequentially coated on the positive electrode current collector, and the positive electrode sheet is obtained after baking and rolling.

[0070] The conductive agent and the binder used to prepare the first slurry and the second slurry may be the same or different.

[0071] In the present invention, the conductive agent, the binder, and the positive electrode current collector may be various materials well known in the art. For example, the conductive agent may be selected from one or more of carbon nanotubes, graphene, carbon black, and carbon fibers. For example, the binder may be one or more of PVDF, PAA, and acrylic acid. For example, the positive electrode current collector may be aluminum foil.

[0072] A fifth aspect of the present invention provides a battery, which includes the positive electrode plate described above.

[0073] In the present invention, the battery can be assembled with positive and negative electrode sheets according to conventional processes in the field. In a specific embodiment, each battery is composed of one electrode core, and each electrode core is composed of 5-8 positive electrode sheets, 7-10 negative electrode sheets and 14-18 separators. Among them, the excess ratio of the negative electrode sheet can be 15%, and the compaction can be 1.60g / cm 3 In a specific embodiment, the negative electrode sheet can be purchased commercially, or prepared according to conventional processes in the art, or prepared by a novel method.

[0074] The battery containing the positive electrode plate provided by the present invention has a better mixed material capacity and better electrochemical performance.

[0075] A sixth aspect of the present invention provides an electrical device, which includes the battery described above.

[0076] The present invention will be described in detail below by way of examples, but the scope of protection of the present invention is not limited thereto. In the following examples, unless otherwise specified, all reagents used are commonly available commercially.

[0077] The lithium iron phosphate secondary particle materials and lithium iron phosphate primary particle materials used in the following examples and comparative examples are all unmodified conventional lithium iron phosphate materials.

[0078] The following Examples A1 to A5 are used to illustrate the preparation process of modified lithium iron phosphate secondary particle materials.

[0079] In the following examples, the particle size D50 is measured by a laser particle size analyzer PSA.

[0080] Example A1

[0081] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:1 and pyrolyzed in a nitrogen atmosphere at 700°C for 6 hours;

[0082] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 100nm.

[0083] Example A2

[0084] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:2 and pyrolyzed in a nitrogen atmosphere at 700°C for 6 hours;

[0085] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 100nm.

[0086] Example A3

[0087] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:3 and pyrolyzed in a nitrogen atmosphere at 700°C for 6 hours;

[0088] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 100nm.

[0089] Example A4

[0090] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:2 and pyrolyzed in a nitrogen atmosphere at 600°C for 6 hours;

[0091] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a primary lithium iron phosphate material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 80nm.

[0092] Example A5

[0093] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:2 and pyrolyzed in a nitrogen atmosphere at 800°C for 6 hours;

[0094] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a primary lithium iron phosphate material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 120nm.

[0095] Comparative Example DA1

[0096] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:0.5 and pyrolyzed in a nitrogen atmosphere at 700°C for 6 hours;

[0097] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 100nm.

[0098] Comparative Example DA2

[0099] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:3.5 and pyrolyzed in a nitrogen atmosphere at 700°C for 6 hours;

[0100] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 100nm.

[0101] Comparative Example DA3

[0102] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:2 and pyrolyzed at 550°C in a nitrogen atmosphere for 6 hours;

[0103] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that a small amount of carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 50nm.

[0104] Comparative Example DA4

[0105] The lithium iron phosphate secondary particle material and melamine were mixed in a mass ratio of 5:2 and pyrolyzed in a nitrogen atmosphere at 850°C for 6 hours;

[0106] Among them, the lithium iron phosphate secondary particle material is composed of the uneven coefficient C u The modified lithium iron phosphate secondary particle material is composed of a lithium iron phosphate primary particle material with a particle size D50 of 1.8, and the particle size D50 of the lithium iron phosphate secondary particle material is 8μm; scanning electron microscopy (SEM) characterization shows that a small amount of carbon nanotubes are formed inside and on the surface of the modified lithium iron phosphate secondary particle material obtained after sintering, and the diameter of the carbon nanotubes is 200nm.

[0107] Comparative Example DA5

[0108] The method of Example 1 was followed, except that the lithium iron phosphate secondary particle material was replaced with lithium iron phosphate primary particle material.

[0109] Specific operations include:

[0110] The non-uniformity coefficient C u The lithium iron phosphate primary particle material with a mass ratio of 1.8 and melamine were mixed in a ratio of 5:1 and pyrolyzed in a nitrogen atmosphere at 700°C for 6 hours;

[0111] The particle size D50 of the lithium iron phosphate primary particle material is 1 μm, as characterized by a scanning electron microscope (SEM). Carbon nanotubes are formed on the surface of the modified lithium iron phosphate particle material obtained after sintering, and the diameter of the carbon nanotubes is 100 nm.

[0112] Test Example 1

[0113] The carbon content, resistivity, and specific surface area of the materials prepared in Examples A1-A5 and Comparative Examples DA1-DA5 were tested respectively. The results are shown in Table 1. The lithium iron phosphate secondary particle materials in Table 1 are composed of the non-uniform coefficient C u The lithium iron phosphate primary particle material is composed of 1.8, and the particle size D50 is 8 μm; the non-uniformity coefficient C of the lithium iron phosphate primary particle material in Table 1 u is 1.8.

[0114] The specific surface area is determined using the BET method. The procedure involves placing a sample in a container and then exposing it to liquid nitrogen. Under these conditions, physical adsorption occurs on the sample surface. A manometer is used to measure the equilibrium adsorption pressure at which adsorption reaches equilibrium, and a volumetric method is used to measure the amount of gas adsorbed by the sample. Finally, the equilibrium adsorption pressure and adsorbed gas volume are substituted into the BET equation to calculate the specific surface area of the sample.

[0115] The test method for the electrode resistivity is as follows: the electrode is uniformly cut into 4cm×10cm samples, the samples are placed in the resistivity test bench, the test pressure is set to 25kPa, the pressure holding time is 40s, and the resistivity test is performed.

[0116] The powder resistivity test method is as follows: 1g of sample powder is placed in a powder resistivity test fixture, the test pressure is set to 0.5t, the pressure holding time is 40s, and the powder resistivity test is performed.

[0117] The carbon content was tested by placing the sample in a crucible, heating it from 100°C to 900°C at a heating rate of 20K / min, and blowing dry nitrogen into the sample.

[0118] Table 1

[0119]

[0120]

[0121] It can be seen from the results in Table 1 that the modified lithium iron phosphate secondary particle material prepared by the method described in the present invention has a low resistivity and an increased specific surface area.

[0122] Resistivity test results show that when the mass ratio of lithium iron phosphate secondary particles to melamine is less than 5:1, the carbon content of the lithium iron phosphate secondary particles is low, and fewer carbon nanotubes are formed within them, failing to effectively improve the resistivity and specific surface area of the lithium iron phosphate secondary particles. When the pyrolysis temperature is too high, some carbon material decomposes, while when the pyrolysis temperature is too low, the carbon material has poor graphitization.

[0123] The following Examples B1 to B5 are used to illustrate the preparation process of the positive electrode sheet.

[0124] Example B1

[0125] (1) The modified lithium iron phosphate secondary particle material prepared in Example A1, a conductive agent (carbon black) and a binder (PVDF) were prepared into a first slurry;

[0126] (2) Lithium iron phosphate primary particle material (uneven coefficient C u is 1.8), a conductive agent (carbon black) and a binder (PVDF) are prepared into a second slurry;

[0127] (3) The second slurry and the first slurry are sequentially coated on the positive electrode current collector (aluminum foil), and after baking and rolling, a positive electrode sheet is obtained, so that the surface density of the single-sided electrode sheet is 250m 2 / g, compacted density is 2.65g / cm 3 .

[0128] Example B2

[0129] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Example A2.

[0130] Example B3

[0131] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Example A3.

[0132] Example B4

[0133] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Example A4.

[0134] Example B5

[0135] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Example A5.

[0136] Comparative Example DB1

[0137] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Comparative Example DA1.

[0138] Comparative Example DB2

[0139] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Comparative Example DA2.

[0140] Comparative Example DB3

[0141] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Comparative Example DA3.

[0142] Comparative Example DB4

[0143] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the modified lithium iron phosphate secondary particle material prepared in Comparative Example DA4.

[0144] Comparative Example DB5

[0145] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the lithium iron phosphate secondary particle material in Table 1.

[0146] Comparative Example DB6

[0147] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the lithium iron phosphate primary particle material in Table 1.

[0148] Comparative Example DB7

[0149] The method of Example B1 is followed, except that the modified lithium iron phosphate secondary particle material prepared in Example A1 is replaced by the material prepared in Comparative Example DA5.

[0150] Test Example 2 Liquid Phase Diffusion Impedance Test

[0151] Two positive electrode sheets and separators prepared in Examples B1-B5 and Comparative Examples DB1-DB7 were sequentially assembled into a core. The core was placed in an outer packaging shell, baked, injected with electrolyte, and then packaged and impregnated to produce a liquid-phase diffusion impedance battery. Liquid-phase diffusion impedance testing was performed using an electrochemical workstation within the frequency range of 300,000 Hz to 0.05 Hz. The results are shown in Table 2.

[0152] Table 2 Electrode resistivity, liquid absorption time, and liquid phase diffusion impedance test

[0153]

[0154]

[0155] It can be seen from the test results in Table 2 that, compared with the comparative example, the positive electrode sheet prepared in the embodiment has both lower resistivity and lower liquid phase diffusion impedance, and has a shorter liquid absorption time, and has better overall performance.

[0156] The following Examples C1 to C5 are used to illustrate the preparation process of the battery.

[0157] Example C1

[0158] Seven positive electrode sheets prepared in Example B1, eight negative electrode sheets (the preparation process of the negative electrode sheet includes: mixing graphite, a conductive agent, a binder, NMP and an aqueous solvent to make the solid content of the slurry reach 45% by volume, stirring to prepare a primer slurry, and then coating it on both sides of a copper foil with a thickness of 8 μm; the copper foil coated with the slurry is baked in an oven, and then rolled and slit to obtain negative electrode sheets.) and 16 separators are wound into a pole core, the pole core is encapsulated with an aluminum-plastic film, the electrolyte is injected, and the battery is obtained through formation, exhaust and aging.

[0159] Example C2

[0160] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Example B2.

[0161] Example C3

[0162] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Example B3.

[0163] Example C4

[0164] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Example B4.

[0165] Example C5

[0166] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Example B5.

[0167] Comparative Example DC1

[0168] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Comparative Example DB1.

[0169] Comparative Example DC2

[0170] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Comparative Example DB2.

[0171] Comparative Example DC3

[0172] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Comparative Example DB3.

[0173] Comparative Example DC4

[0174] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Comparative Example DB4.

[0175] Comparative Example DC5

[0176] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Comparative Example DB5.

[0177] Comparative Example DC6

[0178] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Comparative Example DB6.

[0179] Comparative Example DC7

[0180] The method of Example C1 was followed, except that the positive electrode sheet prepared in Example B1 was replaced by the positive electrode sheet prepared in Comparative Example DB7.

[0181] Test Example 3: Battery Gram Capacity and DCIR Test

[0182] 50% SOC DC internal resistance test method: At room temperature 25±5℃, the battery is discharged at a constant current of 1 / 3C to 2.0V, charged at a constant current of 1 / 3C to 50% SOC, and left for 30 minutes; discharged at a constant current of 1.5C for 30 seconds, and the 50% SOC DC internal resistance is tested.

[0183] Mixed material gram capacity test method: At room temperature 25±5℃, the battery is discharged at 1 / 3C constant current to 2.0V, and charged at 1 / 3C constant current and constant voltage to 3.8V, with a cut-off current of 0.05C, and cycled for 3 cycles; the discharge capacity in the third cycle is the battery capacity, and the mixed material gram capacity = battery capacity / positive electrode dressing amount.

[0184] Table 3

[0185]

[0186]

[0187] It can be seen from the test results in Table 3 that the battery mixture prepared by using the positive electrode sheet provided by the present invention has better specific capacity and better electrochemical performance.

[0188] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A modified lithium iron phosphate secondary particle material, characterized in that: The modified lithium iron phosphate secondary particle material contains lithium iron phosphate secondary particle material and carbon nanomaterial grown inside and on the surface of the lithium iron phosphate secondary particle material, wherein the lithium iron phosphate secondary particle material is composed of a non-uniform coefficient C u The material is composed of lithium iron phosphate primary particles with a density of less than 3.

2.

2. The modified lithium iron phosphate secondary particle material according to claim 1, characterized in that: Based on the total weight of the modified lithium iron phosphate secondary particle material, the content of the carbon nanomaterial is 0.15-2 wt%, preferably 0.3-0.9 wt%.

3. The modified lithium iron phosphate secondary particle material according to claim 1 or 2, characterized in that: The modified lithium iron phosphate secondary particle material has a porous structure and a specific surface area of 13.5 to 15.7 m 2 / g.

4. The modified lithium iron phosphate secondary particle material according to any one of claims 1 to 3, characterized in that: The particle size D50 of the lithium iron phosphate secondary particle material is 6 to 9 μm.

5. The modified lithium iron phosphate secondary particle material according to any one of claims 1 to 4, characterized in that: The carbon content in the modified lithium iron phosphate secondary particle material is 1-3 wt %.

6. The modified lithium iron phosphate secondary particle material according to any one of claims 1 to 5, characterized in that: The carbon nanomaterial is carbon nanotubes and / or carbon nanoparticles; Preferably, the diameter of the carbon nanotubes is 50 to 200 nm; Preferably, the diameter of the carbon nanoparticles is 15 to 40 nm.

7. The modified lithium iron phosphate secondary particle material according to any one of claims 1 to 6, characterized in that: The resistivity of the modified lithium iron phosphate secondary particle material is 3-6Ω·cm.

8. A method for preparing modified lithium iron phosphate secondary particle material, characterized in that: The method comprises: pyrolyzing lithium iron phosphate secondary particles and a carbon source under an inert atmosphere, Wherein, the lithium iron phosphate secondary particle material is composed of a non-uniform coefficient C u <3.2 lithium iron phosphate primary particle material; The pyrolysis temperature is 600-800° C., and the mass ratio of the lithium iron phosphate secondary particles to the carbon source is 5:1-3.

9. The method according to claim 8, characterized in that The pyrolysis time is 5 to 8 hours.

10. The method according to claim 8 or 9, characterized in that The carbon source is melamine.

11. Modified lithium iron phosphate secondary particle material prepared by the method according to any one of claims 8 to 10.

12. A positive electrode plate, characterized in that: The positive electrode sheet includes a first positive electrode active material layer and a second positive electrode active material layer, wherein the first positive electrode active material layer contains the modified lithium iron phosphate secondary particle material according to any one of claims 1 to 7 and 11, and the second positive electrode active material layer contains lithium iron phosphate primary particle material.

13. The positive electrode sheet according to claim 12, characterized in that: The positive electrode sheet further includes a positive electrode current collector, and the second positive electrode active material layer and the first positive electrode active material layer are sequentially covered on one side of the positive electrode current collector.

14. The positive electrode sheet according to claim 12 or 13, characterized in that: The single surface density of the positive electrode sheet is 180 to 300 m 2 / g, compacted density is 2~2.8g / cm 3 .

15. A battery, characterized in that: The battery comprises the positive electrode sheet according to any one of claims 12 to 14.

16. An electrical device, characterized in that: The electric device comprises the battery according to claim 15.

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